The invention relates to a method for determining the NOx concentration in a gas, in particular in the exhaust gas of an internal combustion engine. The concentration is measured with a measuring sensor having a first measuring cell into which part of the gas is introduced and in which a first oxygen concentration is adjusted by means of a first oxygen-ion pumping current, and a second measuring cell, which is connected to the first measuring cell and in which a second oxygen concentration is adjusted by means of a second oxygenion pumping current. The NOx concentration is measured with a measuring electrode in the second measuring cell.
For measuring the NOx concentration in a gas, for example in the exhaust gas of an internal combustion engine, it is known to use a thick-film measuring sensor. Such a measuring sensor is described in the publication by N. Kato et al., xe2x80x9cPerformance of Thick Film NOx Sensor on Diesel and Gasoline Enginesxe2x80x9d, Society of Automotive Engineers, publication 970858, 1997. This measuring sensor has two measuring cells and consists of a zirconium oxide that conducts oxygen ions. It realizes the following measuring concept: in a first measuring cell, which is fed the gas to be measured via a diffusion barrier, a first oxygen concentration is set by means of a first oxygen-ion pumping current, with no decomposition of NOx taking place. In a second measuring cell, which is connected to the first measuring cell via a diffusion barrier, the oxygen content is further reduced by means of a second oxygen-ion pumping current and NOx decomposes at a measuring electrode. The oxygen generated in this way is sensed as a measure of the NOx concentration. The entire measuring sensor is in this case brought to an elevated temperature, for example 430xc2x0 C., by means of an electric heater. The measuring error of the measuring sensor described in this publication corresponds to an NOx concentration of 22 ppm.
The measuring error applies to steady-state operation, i.e. when no great changes in concentration are to be measured. A residual oxygen content in the second measuring cell, which does not originate from a decomposition of NOx, leads to this measuring error, since the oxygen in the second measuring cell is taken as a measure of the NOx concentration.
In addition, the measuring error of the NOx measuring sensor described is strongly temperature-dependent. The cause of this is that the first measuring cell is connected to the gas to be measured via one diffusion barrier and is connected to the second measuring cell via a second diffusion barrier. The diffusion through these diffusion barriers is temperature-dependent, which leads to a temperature-dependent residual oxygen content in the second measuring cell. According to the prior art, it is attempted to remedy this by choosing the desired concentrations in the two measuring cells such that the second oxygen-ion pumping current for setting the oxygen concentration in the second measuring cell can be controlled to a fixed value, at which there is greatest possible temperature independence.
The object of the invention is to provide a method of determining a NOx concentration in a gas which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which allows more exact sensing of the NOx concentration in a gas using the measuring sensor described above.
With the above and other objects in view there is provided, in accordance with the invention, a method of determining a NOx concentration in a gas, which comprises:
providing a measuring sensor with a a first measuring cell and a second measuring cell connected to the first measuring cell;
introducing a portion of a gas into the first measuring cell and adjusting a first oxygen concentration with a first oxygen-ion pumping current;
adjusting a second oxygen concentration in the second measuring cell with a second oxygen-ion pumping current;
measuring a NOx concentration with a measuring electrode in the second measuring cell;
correcting the measured value of the NOx concentration by a multiplicative correction value and an additive correction value, the correction values being taken from a characteristic map in dependence on at least the second oxygen-ion pumping current.
The novel method is particularly well suited for application in automotive exhaust systems. In that case, the sensor is exposed to the exhaust gas of an internal combustion engine and the NOx concentration in the exhaust gas of the internal combustion engine is measured.
In accordance with an added feature of the invention, the characteristic map is defined in a test bed measurement of the measuring sensor and thereby varying at least one parameter selected from the group consisting of a NOx concentration, a rate of change of the NOx concentration, an O2 concentration, and a rate of change of the O2 concentration.
In accordance with an additional feature of the invention, the characteristic map is defined in a test bed measurement of the measuring sensor under predetermined test gas conditions with the second oxygen-ion pumping current Ip1 being adjusted over a certain range.
In accordance with another feature of the invention, the characteristic map is additionally also dependent on a temperature of the measuring sensor and in addition, or alternatively, it is also dependent on a temperature of the gas to be measured.
In accordance with a concomitant feature of the invention, the temperature of the measuring sensor is the temperature at the second measuring cell.
In other words, according to the invention, the raw measured value supplied by the NOx measuring sensor is corrected by a multiplicative correction value f and an additive correction value a. Both correction values are taken from a characteristic map, which was preferably determined from the test bed measurement of the measuring sensor under specific conditions, in particular at various temperatures, NOx concentrations, rates of NOx change etc., and which depends at least on the second oxygen-ion pumping current, preferably in addition on the temperature of the gas to be measured and/or the temperature of the measuring sensor, since the residual oxygen content causing a measuring error depends on the second oxygen-ion pumping current Ip1. The improved measured value is then obtained by the following equation:
NOx(improved measured value)=fxc2x7NOx(raw measured value)+axe2x80x83xe2x80x83(I).
The dependence of the correction factors on the oxygen-ion pumping current in the second measuring cell, which is a measure of the residual oxygen content in the second measuring cell, causing the measuring error, and preferably in addition on the temperature of the sensor, which is a measure of the diffusion through the diffusion barriers, and the gas temperature, as well as the evaluation of this dependence for the correction of the measured NOx raw measured value make it unnecessary furthermore for the second oxygen-ion pumping current to be controlled to a constant value.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for determining NOx concentration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.